Automated photographic film handling processes, such as film scanning and printing processes, require the ability to accurately position the film at the location of each exposure (image frame). For example, photofinishing systems customarily prescan a strip of (35 mm) color film to examine the quality and color content of the respective image frames prior to transferring the images to print paper. On the basis of this prescan examination, exposure control parameters of the scanning optics components are defined so that, during a second pass of the film, the projection of each image onto the print medium will be properly adjusted to yield an acceptable quality hard copy print.
In order to accurately identify where each image is located, it is conventional practice to form respective notches along the edge of the film, for example between successive frames, or centrally of each frame. When the film is rescanned, these notches are used to identify successive frames for scanning of optical printing. A shortcoming of such a control procedure is the fact that a notch is sometimes missed or mislocated. When this happens, a mismatch between the current image frame and prescan-derived exposure control parameters can occur. The result is a poor quality set of prints, making it necessary for the photofinisher to reprocess the film strip, which entails additional time, and costs associated with the wasted print materials. Although a minor lag in processing can be adequately managed, when a large reel of film is to be scanned continuously on a single machine and then printed on the same (or other) machine, it is too complex to track and detect possible sequence errors.
One attempt to remedy this notch misalignment problem, described in the U.S. Patent to Benker et al, U.S. Pat. No. 4,947,205, is to make use of the film density change occurring at interframe gaps between successive image frames. In accordance with the system described in the '205 patent, film density change measurements are conducted in order to compensate for `slippage` between the film strip transport reels and the film strip itself. A counter is operative, in conjunction with film transport notches along the sides of the length of the film, to provide an indication of film advance, and it is assumed that well formed image frames are properly spaced apart by periodically spaced (Dmin) interframe gaps. When the film strip is rewound in preparation for a rescan, the output of a film density monitoring circuit, which looks for interframe gaps, is compared with the notch counter. If the two do not coincide, then it is inferred that slippage has occurred and the position of the film is adjusted to compensate for the difference.
Unfortunately, it is not always the case that successive images on a continuous film strip are equally spaced apart from one another, nor is it always the case the content of an image frame immediately adjacent to an interframe gap is at a signal modulance level different from Dmin. Indeed, it often occurs that two or more image frames on a film strip to be processed overlap each other (a condition that may occur if the film has been rewound by the camera user, for example). Also, the captured image (modulance) contents of an image frame may be such that the modulance cannot be distinguished from non-exposed (interframe-gap) portions of the film (for example, a night-time fireworks image, where the principal portion of the image frame surrounding the fireworks display is effectively in darkness). In this latter case, the interframe gap and the image frame itself effectively blend together.